Which pins of the aircraft service cable must be connected for the 400 Hz external power interlock to energize the output contactor?

• A. E and F
• B. A and B
• C. C and D
• D. G and H

Answer: (A)

The key idea is that the 400 Hz external power interlock is a two-wire control path. The output contactor will energize only when that interlock circuit is complete, which happens when the two specific interlock pins are connected to the external power source. Those two pins provide the necessary 400 Hz signal and return path to energize the coil of the output contactor. If either interlock pin is left open, the coil doesn’t energize and the contactor stays de-energized, so external power cannot be applied to the aircraft systems.

Pins chosen for the interlock must form that complete circuit; other pin pairs may carry the main power or other signals but do not complete the interlock coil path, so they won’t energize the output contactor. Therefore, the interlock requires the two designated pins to be connected to function as intended.